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FBI analysts gave testimony that contained serious scientific errors in hundreds of trials over the course of two decades, including 29 in Pennsylvania, the Justice Department announced Monday.

The FBI and Justice Department, alongside the Innocence Project and the National Association of Criminal Defense Lawyers, have been conducting a review of cases where “microscopic hair comparison analysis” — a process in which analysts examined hair found at crime scenes to try and match its color and patterning to a suspect — was used as evidence. Prior to 2000 — at which point DNA testing of hair became routine — the FBI relied heavily on this kind of analysis. Nowadays, the National Academy of Sciences considers microscopic hair comparison analysis “highly unreliable.” So far, the review has looked at 268 trials, and found that 257 of them — over 90 percent — contained incorrect testimony.

Twenty-nine of those trials took place in Pennsylvania. The groups conducting the review are currently not releasing the names of the defendants in those trials, saying that defendants and lawyers should be able to make their own decision about going public. The Innocence Project confirmed, however, that a large portion of the defendants were on trial for sexual assault or murder.

Six of the trials in Pennsylvania sent people to death row. (Other reports had that number as 5, and the Pennsylvania total at 28, but Paul Cates at the Innocence Project confirms that an additional death penalty case was discovered subsequently.) As of now, all of those defendants are still alive, according to the Innocence Project. But it’s a sure thing that death penalty opponents will point to Monday’s announcement as further damaging the case for the death penalty and justifying Governor Tom Wolf’s moratorium.

“What is shocking about this is that there was a systemic use of unreliable testimony used to implicate defendants,” said Robert Dunham, executive director of the nonpartisan Death Penalty Information Center. “It is part of an emerging pattern that shows that there is an enormous amount of inaccurate testimony in our courtrooms.”

The FBI review is ongoing, and the agency plans to examine thousands more cases in the coming months. That might just be the tip of the iceberg, however. The FBI says that it helped train police departments all over the country, including Pennsylvania, in microscopic hair comparison analysis. It’s hard to know how many prosecutions used that analysis as evidence, but with several hundred of investigators using this technique over more than 20 years, it’s certainly a lot, according to Dunham, and there’s no guarantee every single error will be uncovered. “Even if we find out about some of these cases, we’re going to find out too late for some people,” he said.